Abstract Prosthetic joint infection (PJI) is the bane of elective total joint replacement (TJR) surgery, of which the vast majority is caused by Staphylococcal species. The 1-5% incidence of PJI is known to be a nonrandom event that is largely determined by patient specific factors. Moreover, ~13% of patients infected with S. aureus become septic and die from multiorgan failure, while others recover with little intervention. In addition to the established host susceptibility factors, there are two leading theories to explain this. The first is the ?immune proteome? hypothesis, which posits that the array of specific antibodies a host develops against S. aureus dictates its susceptibility vs. protection to infection. The second is the ?Trojan horse? model of S. aureus dissemination, in which leukocytes get infected at the surgical site, and transport intracellular bacteria to the blood stream and internal organs, culminating in septic death of susceptible hosts. Currently, there is no direct experimental or clinical evidence to support these theories. However, we recently made several potential breakthrough discoveries that substantiate these models. The first is that antibodies against the autolysin (Atl) proteins efficiently induce megacluster formation, opsonophagocytosis, and protection/survival following S. aureus osteomyelitis in mice and humans. The second is that iron sensing determinant B (IsdB) is the most immuno-dominant S. aureus antigen in mouse and man, and that antibodies against IsdB are associated with sepsis, multiorgan failure and death following surgical site infection.8,13 Here we propose to: 1) elucidate the molecular mechanism of anti-IsdB mediated septic death, 2) validate the ?susceptible? anti-Isd vs. ?protective? anti-Atl immune proteome in PJI patients, and 3) extend this osteoimmunology to elucidate the immune proteomes of S. epidermidis and S. lugdunensis. Our approaches to achieve these goals are embodied by three Specific Aims. In Aim 1 we will elucidate the mechanism of anti-IsdB and related anti-IsdA and anti-IsdH antibody mediated sepsis following S. aureus osteomyelitis. We will also complete a clinical pilot study to demonstrate that osteomyelitis patients who succumb to S. aureus septic death have high anti-Isd titers and Trojan horse macrophages in their blood and internal organs. In Aim 2 we will test the immune proteome hypothesis by correlating anti-Isd and anti-Atl titers with the clinical outcomes of 300 PJI patients that undergo 2- stage revision surgery using our multiplex-Luminex assay, and define the susceptible:protective index of these immune proteomes. And in Aim 3 we will extend our knowledge of the mammalian host response to Staphylococcus species by generate multiplex-Luminex assays for S. epidermidis and S. lugdunensis, and uses them to screen sera from mice and PJI patients as we have done for S. aureus. !